I haven’t seen a commercial kit that I really liked, but have not spent much time looking for one. The suggestions from others are good, and what you need depends on both your training and the probability of encountering situations, as well as portability, cost and the ability to get and use items. This list is more extensive and may not apply to most people.
If you have the skills, there may be merit to a stethoscope, blood pressure cuff, thermometer in necessary range, blood sugar monitor, flashlight for eye and mouth exams and darkness, oxygen saturation monitor and, much less importantly, ability to record a rhythm strip or an ophthalmoscope. A handheld ultrasound is much pricier and requires much more skill, one is unlikely to need it without professional experience.
Assuming physical injuries are common, it makes sense to have (if you know how to use) gauze, bandages, tourniquets, splints, plaster with foam roll, an LMA, tongue depressors, something to measure general pH, a compact medical text, possibly syringes (depending on medications), stuff like cotton for cleaning, disinfectants, sutures and a disposable suture kit or scissors, tweezers and clamps, a ruler and phone with camera and communication abilities, plastic bags, gloves, mask, blanket, (and for car use) traffic cone, signal flares, shovel, whistle, and stuff for snow and ice if relevant. Ambulances carry cervical collars and backboards, but these things are likely beyond what most need or know how to use. Things for boats might differ and include other rescue equipment.
Medications may be helpful. Basic stuff includes sugar packets, antihistamine, antibiotic ointment, sterile disinfectant, pain medicines including aspirin, medicines for nausea and vomiting (preferably in both pill and suppository form, but pills can be used in that way too), a small bottle of booze, and medicines specific to needs. Prescription stuff may include (with training) local anesthetic, anxiolytics, antibiotics, stronger pain medicines, prednisone, salbutamol puffer, epinephrine, naloxone, medicines specific to mountain climbing/diving/antidotes/oxygen as relevant, medicines specific to you and your companions, possibly other things such as airway adjuncts and heart and other medications with specific need and training. Ipecac is out of fashion, I doubt I would include it.
I don’t want to get into recent basic life support protocols, but believe rescue breathing can be helpful. I wouldn’t carry a specific mask for it, and might not use it in certain cases. The fact it is a mix of oxygen and carbon dioxide does not mean it can’t be helpful or mixed with compressions. But in practice things like amount of available help, patient and situation specifics, and time to get help certainly might make it more or less useful, and out-of-hospital resuscitations do not always go well.